1. Technical Field
The present invention relates to semiconductor devices. More particularly, the present invention relates to the fabrication of silicon bipolar junction transistors.
2. Description of the Prior Art
Rapid progress has been made in improving silicon bipolar transistor technology in recent years. As a result of such improvement, silicon bipolar junction transistors are widely used for various circuit applications. Some of the areas in which these improvements have been made include, for example, reduced line width lithography, lateral transistor scaling, defect density reduction, and advances in silicon integrated circuit technology, such as multilevel interconnect. Accordingly, it is not unusual for bipolar device integration levels to reach 100 k gates/chip.
At the same time, advances in bipolar device technology have permitted vertical, as well as lateral, device scaling and reductions in parasitics that have pushed minimum gate delays down to 20 ps (unloaded) and transistor gain up to 52 GHz. See, for example, S. Chiang, D. Pettengill, P. Vande Vorrde, Bipolar Device Design For Circuit Performance Optimization, IEEE 1990 Bipolar Circuits and Technology Meeting.
A bipolar transistor is an active device having three elements: an emitter, a collector, and a base. The structural inter-relation of these elements in an integrated circuit is shown in FIG. 1, in which a bipolar transistor 10 is shown in cross section as part of an integrated circuit. The structure shown is a typical NPN-type device in which the emitter 16 and collector 12 have an N-type doping and the gate 14 has a P-type doping. It should be appreciated that bipolar transistors may be formed in many different ways, such that the appearance of some bipolar transistors may not exactly match that shown in FIG. 1. For example, in a PNP device, the emitter and collector have a P-type doping and the base has an N-type doping.
In FIG. 1, an NPN bipolar transistor 10 is shown having a collector 12 and an emitter 16. A base 14 is formed between the collector and the emitter. In applications requiring a fast transit time it is desirable to have a narrow base. Therefore, the base typically includes an active portion, referred to as the intrinsic region 17, and a portion that serves to connect the intrinsic base to a metal interconnect system, referred to as the extrinsic region 15. The bipolar transistor structure is represented schematically in FIG. 1a.
Bipolar transistors are attractive design elements because of their high speed. One characteristic measure of this speed is the transit time (".tau..sub.EC ") across the base. A dominant portion of .tau..sub.EC is .tau..sub.B, the base transit time. Thus, it is common practice to taper the base at the intrinsic region such that the width of the base W.sub.B is much narrower than the extrinsic base region. This tapering is shown in FIG. 1. Tapering the base, while initially improving device speed, has been found to increase resistance across the base. Thus, the extrinsic base region is usually designed with a lower resistance than the intrinsic base region, i.e. R.sub.BX &lt;R.sub.BI. One way of viewing this phenomenon is by analogy to a water pipe. A narrower pipe passes less water than a wider pipe in the same amount of time and therefore offers more resistance to the flow of water.
It is therefore desirable, for reasons of speed and performance, to have both W.sub.B and R.sub.Bx small. This implies the need to provide a high level of doping in the extrinsic base region, which can lead to two problems. If an extrinsic-intrinsic base link is provided that is too shallow, then contact to the intrinsic region is poor or non-existent, or the high doping in that region gives a very leaky emitter-base diode that reduces .beta., current gain, and degrades emitter-base ("EB") diode characteristics. If the extrinsic-intrinsic base link is too deep, the capacitance between the collector and the base ("C.sub.CB ") is increased and breakdown voltage is decreased. Also, the effect upon transit time .tau..sub.EC across the base of the transistor increases because the intrinsic base is effectively wide (and slow) at the emitter edge. The transit time .tau..sub.EC is also increased by capacitive terms as the extrinsic base or link doping level is increased.
Various techniques for making an electrical connection from an extrinsic base region to a narrow intrinsic base region in a bipolar transistor have been described in the art. Self alignment schemes for emitter-base structures have been suggested as a partial solution to the problem of fabricating low resistance intrinsic base structures in bipolar transistors. For example, it has been proposed to use an ultra-shallow link base in a double polysilicon transistor. See J. D. Hayden, J. D. Burnett, J. R. Pfester, M. P. Woo, An Ultra-Shallow Link Base for a Double Polysilicon Bipolar Transistor, IEEE 1992 Bipolar Circuits and Technology Meeting. In the technique described in the above paper, boron is diffused through a thin oxide. This leads to control problems because the link profile is strongly affected by the oxide thickness which cannot be well controlled.
It has also been suggested that the intrinsic-extrinsic base structure could be optimized by the use of ion implantation. However, it is difficult to scale an ion implant to a shallower depth to minimize intrinsic base region thickness W.sub.B. At lower energies, much of the implant is lost in the screen oxide typically used to reduce stray contaminants. As the total concentration implanted (at lower energy) is increased to compensate for the fraction lost, a similar "active" profile is produced. A very low implant dose ("integrated concentration") is also necessary to achieve a shallow implant depth, such that the resulting base structure exhibits the emitter-base profile necessary for improved device performance. A dose low enough to provide a shallow implant profile would necessarily be so low that it would cause a high link resistance (high R.sub.BX). Thus, the available range of implant profile depth does not scale with a reduction in implant energy.
Vapor phase doping is another approach suggested for achieving a shallower base structure. However, such approach produces an undesirable variability in the dose due to variations in native surface oxide which control the depth of diffusion.
The problem of optimizing bipolar transistor structures for speed by improving base characteristics has been difficult to solve. Most approaches to fabricating bipolar transistors have accepted the limitations imposed by known intrinsic-extrinsic base link structures. However, it is not only difficult to use such known approaches to produce faster devices, but as device features become smaller and smaller, it becomes increasingly difficult to produce a scalable intrinsic-extrinsic base structure. As a result, device density is limited by the present state of the art. Thus, there is presently no way to optimize bipolar transistor intrinsic-extrinsic base structures for maximum speed and/or minimum size.